Powder Coating Props on BIII

[wickeral]

As is widely reported, Corrosion and Upper Bearing/Gear Failure are reportedly an Issue. I have a BIII on our 97/250, (5200LBS.) and am quite comfortable with it. I've installed a Drive Shower which I consider a must for BIII's and have Powder Coated the Props for any Electrolysis potential. (I can't explain why, but the Powder Coated Props gave me 200 additional RPM.). To me that equates that the Engine isn't Bogged down on the Torque Generated, so it's a big plus! How about a few comments on this? Al W.

 

[Presentation]

Painting the stainless propellers to reduce galvanic corrosion seems interesting. I’d like to see if other members have a corrosion education background stronger then mine to see if there is any issue with this. If it’s a good idea I wonder why mercury does not do it. Stainless steel propellers go thru a fair amount of flexing. How well has the paint remained adhered to the propeller after use? Yes, I understand that you are talking powder coating / baking so it’s not technically paint.

I would argue that you would not reduce electrolysis via propeller painting. The electrical potential, if present, would not be generated via the stainless steel propellers and the electrons would still go to the least resistive component.

 

[Four Suns]

I have a whole bunch of powder coated hatch frames that I replaced last year where the aluminum corroded underneath and the powder coat bubbled up and flaked off. Powder coating is also very brittle and will not tolerate flexing and will nick when it hits something... and oxygen will get in underneath at that point...

 

[dpvandy01]

One of my firefighters has the knowlege and ability to powder coat and he tells me that there is an addative that will allow for small amounts of flex similiar to the addatives for high temprature applications such as engine, exhausts etc.... He's going to look into it for me. I'll advise.

 

[Searay Jim]

I would agree with Presentation. I've never heard of or seen SS props corrode. Cavitation burn yes, I battled that, but that's a whole other issue.

And I'm really scratching my head tryig to figure out how adding thickness and weight, al biet very little, would give you 200 RPM more. That's like going down 2" in pitch. Unless in the process, something happened that has lead to increasing slip. You'd need to know all the basics WOT RPM's, GPS speed etc. both before and after in the same conditions to know though.

Are you sure the air temp isn't a lot cooler than before? That would account for 100-200 RPM more.

 

[wickeral]

OK, here's some more to go on. I'm working with my friend on this and he is one of the top Prop Techs on the East Coast. He owns a very reputable shop and also Manufactures a widely used Zinc Anode. We've been working on this as an experiment
focusing on Electrolysis and the BIII specifically.
Our Theory is to remove all exposed metal surfaces on the BIII from direct contact with the water. With this accomplished any current transfer will be done mostly by the Anodes. The poor Nobility of quality Zinc will cause them to sacrifice as desired.
Dupont supplied us with the Powder Coat System which was applied by a commercial manufacturing plant owned by another friend. It consisted of Primer and Finishing material. We Etched the props after sand blasting and chemically cleaned them before coating. They still look like new after 43 hours of use. (and no sand plowing). Al W.

 

[boatrboy]

Al - I am very interested in your findings. Can you let us know where you are? what kind of water? What shops are you working with?
Mark

 

[Presentation]

OK, I re-read the entire string. From what I am reading it seems possible there is a misunderstanding on what causes galvanic corrosion and what causes electrolysis. These are two very different issues and are caused by two very different reasons.

Let’s say you had no paint on any surface on one drive and everything painted save the anodes on another. Galvanic corrosion would be very high on the unpainted drive and low on the painted one. However, all else being equal, the existence or lack of electrolyses would be unaffected.

 

[wickeral]

OK, I re-read the entire string. From what I am reading it seems possible there is a misunderstanding on what causes galvanic corrosion and what causes electrolysis. These are two very different issues and are caused by two very different reasons.

Let’s say you had no paint on any surface on one drive and everything painted save the anodes on another. Galvanic corrosion would be very high on the unpainted drive and low on the painted one. However, all else being equal, the existence or lack of electrolyses would be unaffected.

More to go on. Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially when it is in contact with a different type of metal and both metals are in an electrolyte.

When two or more different sorts of metal come into contact in the presence of an electrolyte a galvanic couple is set up as different metals have different electrode potentials. The electrolyte provides a means for ion migration whereby metallic ions can move from the anode to the cathode. This leads to the anode metal corroding more quickly than it otherwise would; the corrosion of the cathodic metal is retarded even to the point of stopping. The presence of electrolyte and a conducting path between the metals may cause corrosion where otherwise neither metal alone would have corroded.
Wikipedia gets credit for this explanation!
My Interpretation of this is one is the result or reliant on the other. We can chase our Tails all day on this as a Debate but my purpose here is to Help my Fellow BIII owners avoid preventable problems. Simply put. In the case of our boats, Galvanic Corrosion is reliant on the water conducting current, (Electrolysis). Naturally Salt Water is a better conducter than fresh. Al W.

 

[Dave S]

Ever since I read your first thread sometime back I have been interested in trying the powder coating on the props. I would think that if you can somehow show the boating populace at large that such a treatment greatly reduces corrosion, your friend the prop tech could add that service to his business.

In affect I really think Mercury has done the same thing with the Sea Core drives. Only they treat the whole drive case which eliminates the electrolysis action from occurring on the aluminum case. In your situation, by treating the prop you have reduced the number one cause of electrolysis on a standard B3 drive and that is the massive amount of raw stainless concentrated in one place on the drive. As such there is little SS left to interact with the standard aluminum casing of the regular Bravo Drive.

You may recall me mentioning that we have a particularly bad corrosion problem on our lake. Mercury has sent techs to the lake and they can't figure out why all the Bravo 3 equipped boats on our lake have this problem. It's a lakewide issue and not confined to one particular location or Marina. Here is a picture of my outdrive I took last week after my emergency haul out because of low water.

All of this corrosion occured in five months time because I had refinished the drive at that time and it looked like this.

Here is a shot of the drive before I refinised it in May. The boat had been in the water for nine months then.

I have the additional Mercathode pucks on the transom as well which you can see. Because they didn't do the job, I complained loudly about this to my dealer who in turn complained to Mercury and they authorized a heavy duty mercathode controller and different set of pucks that will be installed now with the boat out of the water. I think the pucks do work because I have no other corrosion around the other stainless on the outdrive whereas boats without the pucks do corrode in many other areas where there is stainless. It's just that I think these pucks cannot overcome (at least in our lake water) the effect of the surface area of those huge chunks of stainless steel (props) and as such most of the corrosion is confined to the prop hub and skeg. I really don't think the new controller and pucks will solve the problem though and I would certainly welcome a more permanent solution. It's just that I don't want to have scout up someone to find the coating and then apply it. So if you or your friend decides to do this as a business, let me know. I can guarantee you I will give a lot of referrals to others on our lake who are dealing with this same problem I have. Even our dealer would welcome a solution so they may be willing to offer the service to customers too.

By the way, I will be refinishing the drive again next week and installing all new anodes so once the boat goes back in the water (whenever that is with our low water situation) I will keep an eye on the drive and can let you know if the corrosion problem is reduced with the newer Mercathode.

 

[Sorrento 25]

Sorry if anyone considers this off-topic but why do stainless props accelerate corrosion on adjacent metals?

 

[dpvandy01]

Sorry if anyone considers this off-topic but why do stainless props accelerate corrosion on adjacent metals?

"I'm not a smart man.... jenny" sorry for the forest gump quote but i understand it to be the fact that dissimilar metals and electric current produced either through leaking from the marina/boat or current (water) passing through these metals causes electrolysis. Since the aluminum out drive is a softer metal than the Stainless prop.. the outdrive suffers???? All you techno-geeks (superior education) can explain this to a much greater and far more accurate degree than myself, I'm quite sure...

 

[Dave S]

"I'm not a smart man.... jenny" sorry for the forest gump quote but i understand it to be the fact that dissimilar metals and electric current produced either through leaking from the marina/boat or current (water) passing through these metals causes electrolysis. Since the aluminum out drive is a softer metal than the Stainless prop.. the outdrive suffers???? All you techno-geeks (superior education) can explain this to a much greater and far more accurate degree than myself, I'm quite sure...

That is essentialy correct. But the anodes on your outdrive should desinigrate first (theoretically) rather than the aluminum outdrive when you use a Mercathode System. Maybe this will explain it better. http://www.mercstuff.com/corrosion.htm

 

[wickeral]

DaveS, Thanks for taking the time and giving your experience and observations. one quick question! Are those Anode Pucks on your Transom Bonded to the Engine or Battery Circuit of your boat. If not they are useless at best.
It's obvious that your Lake is a Harsh environment, If it is a "Common" Occurrence for BIII's to show Galvanic Corrosion of the Aluminum there will be a "Common" Cure. On my boat, 1997 Dancer,7.4/300HP with a BIII I have installed the recommended "Second" Cathod Amplifier or as Mercury refers to it a "Mercathode". Your boat is delivered with one in place, it is either blue or black. Dave, you and everybody on your Lake need to double up the Mercathode now! No need to add the Anode portion just the Blue or Black Four Wire Unit.
Also, all those little Stainless Steel Ground Bond wires on the Drive must be in Tip Top condition.
Getting back to our boat. I also painted the Gimbal and Drive. I used Dupont Imron, which is a Two Part, or Catalyzed coating system widely used in Aviation, Marine and Fleet Applications. Why? Because it is my belief that it does a better job of "Insulating" the Drive than the Mercury recommended Tin based coating. And, for guys like DaveS and I where appearance is important you can TopCoat with Clear for that Deep Black Shine! (I like it!).
Also, an area at least 1" from the Gimbal Housing on your Hull must be free of Bottom Paint. Al W.
Here's more: http://www.sterndrives.com/bravo_3_corrosion.html

 

[Presentation]

Dave - the anode aft of the propellers, is that Zink or aluminum? Have you considered magnesium for this anode?

In your photos, the anodes appear to be dull, not shiny. This would indicate that you have a galvanic corrosion issue, not electrolysis.

Since your drive appears to be under protected, possibly caused by water that has rare combination of being less conductive yet as corrosive as pure salt water, it would be something to try.

You would go thru magnesium anodes faster then Zink or aluminum. Magnesium is less noble then Zink and would increase the sacrificial nature of the anode.

Magnesium, due to its relatively low nobility, can restore the anode corrosion protection even in moderately conductive fresh water.

 

[Presentation]

Sorry if anyone considers this off-topic but why do stainless props accelerate corrosion on adjacent metals?

Hi Mike,

Here is a basic nobility chart from more to less.

Stainless steel
Bras
Aluminum
Zinc
Magnesium

Put one in contact with the other and the less noble metal becomes the anode.

Stainless steel is very noble.

I think the idea here is that by painting the stainless steel propellers you reduce galvanic corrosion.

I still don’t by into painting the propellers having any effect on electrolysis.

 

[Dave S]

Dave - the anode aft of the propellers, is that Zink or aluminum? Have you considered magnesium for this anode?

All the anodes are currently Magnesium. The dealer replaced the original aluminum ones before the boat was ever put in the water last year. they are pretty badly corroded which is what you would expect after 14 months in the water. I have a brand new set of Magnesium ones and will be replacing all the anodes on the outdrive and hydraulic rams.

 

[wickeral]

I have a whole bunch of powder coated hatch frames that I replaced last year where the aluminum corroded underneath and the powder coat bubbled up and flaked off. Powder coating is also very brittle and will not tolerate flexing and will nick when it hits something... and oxygen will get in underneath at that point...

Hi Gary, Sorry I didn't comment on your Post sooner, but here goes. Powder Coating is subject to expertise or Manufacturers Process for Quality or Longevity. We use Powder Coating on many different Metals and for Exterior use. As with Paint, you can Buy Interior Grade and Exterior Grade. Dupont even has an Marine Grade. You can also Buy Gloss, Semi Gloss Etc.. If you Skip Primer when called for the result you describe will result. As for the comments about Flexing of the Props, properly prepared surface will tolerate a minimal amount of Flex. Al W.

 

[Sorrento 25]

Thanks Doug. That makes sense, but this thread is leading me to believe that stainless props seem to accelerate the process in some way. Is the aluminum drive housing behind the prop going to corrode any less with an aluminum prop than a stainless one all other factors being the same?

I believed that a stainless prop was simply the item least likely to corrode, and the more metal you can upgrade in "nobility" the less there is to fight with the zincs for their share of the corrosion pie.

I powdercoat perforated aluminum mesh for an automotive application and it is very flexible. We use a simple exterior grade DuPont powder with no primer or special preparation and the powdercoat will probably flex to at least a 3" radius before it cracks. If the application is thicker it will crack sooner.

 

[Four Suns]

I'm not a powder coat guy so you are probably right on the flexing and grade of powder coat. I believe my frames going bad were due to dismaler metal issues between the stainless screws and hinges used with aluminum frames but that's not on topic here... other than it doesn't take much to corrode aluminum.